The story details the events of a disastrous expedition to the Antarctic continent in September 1930, and what was found there by a group of explorers led by the narrator, Dr. William Dyer of Miskatonic University. Throughout the story, Dyer details a series of previously untold events in the hope of deterring another group of explorers who wish to return to the continent.

The story is told in a first-person perspective by the geologist William Dyer, a professor at Arkham's Miskatonic University, in the hope to prevent an important and much publicized scientific expedition to Antarctica. Throughout the course of his explanation, Dyer relates how he led a group of scholars from Miskatonic University on a previous expedition to Antarctica, during which they discovered ancient ruins and a dangerous secret, beyond a range of mountains higher than the Himalayas.

In Dyer's story, a smaller advance group, led by Professor Lake, discovers the remains of fourteen prehistoric life-forms, previously unknown to science, and also unidentifiable as either plants or animals. Six of the specimens have been badly damaged, while another eight have been preserved in pristine condition. The specimens' stratum places them far too early on the geologic time scale for the features of the specimens to have evolved. Some fossils of Cambrian age show signs of the use of tools to carve a specimen for food.

When the main expedition loses contact with Lake's party, Dyer and his colleagues investigate. Lake's camp is devastated, with the majority of men and dogs slaughtered, while a man named Gedney and one of the dogs are absent. Near the expedition's campsite, they find six star-shaped snow mounds with one specimen under each. They also discover that the better preserved life-forms have vanished, and that some form of dissection experiment has been done on both an unnamed man and a dog. The missing man is suspected of having gone utterly insane and having killed and mutilated all the others.

Dyer and a graduate student, named Danforth, fly an aeroplane across the mountains, which they identify as the outer walls of a vast abandoned stone-city, alien to any human architecture. For their resemblance to creatures of myth mentioned in the Necronomicon, the builders of this lost civilization are dubbed the "Elder Things". By exploring these fantastic structures, the men learn through hieroglyphic murals that the Elder Things first came to Earth shortly after the Moon took form and built their cities with the help of "shoggoths" — biological entities created to perform any task, assume any form, and reflect any thought. There is a hint that all earthly life evolved from cellular material left over from the creation of the shoggoths.

As more buildings are explored, the explorers witness the Elder Things' conflict with both the Star-spawn of Cthulhu and the Mi-go, who arrived on Earth shortly afterwards. The images also reflect a degradation of their civilization, once the shoggoths gain independence. As more resources are applied in maintaining order, the etchings become haphazard and primitive. The murals also allude to an unnamed evil lurking within an even larger mountain range located beyond the city. This mountain range rose in one night and certain phenomena and incidents deterred the Elder Things from exploring it. When Antarctica became uninhabitable, even for the Elder Things, they soon migrated into a large, subterranean ocean.

Dyer and Danforth eventually realize that the Elder Things missing from the advance party's camp had somehow returned to life and, after slaughtering the explorers, have returned to their city. Dyer and Danforth also discover traces of the Elder Things' earlier exploration, as well as sleds containing the corpses of both Gedney and his missing dog. They are ultimately drawn towards the entrance of a tunnel, into the subterranean region depicted in the murals. Here, they find evidence of various Elder Things killed in a brutal struggle and blind six-foot-tall penguins wandering placidly, apparently used as livestock. They are then confronted by a black, bubbling mass, which they identify as a shoggoth, and escape. Aboard the plane, high above the plateau, Danforth looks back and sees something which causes him to lose his own sanity, implied to be the unnamed evil itself. Dyer concludes the Elder Things slaughtered the survivors and dogs only out of self-defense or scientific curiosity, that their civilization was eventually destroyed by the shoggoths and that this further entity has preyed on the enormous penguins. He warns the planners of the next proposed Antarctic expedition to stay distant from the site.

William Dyer (ca. 1875–?): The narrator of At the Mountains of Madness, he is a professor emeritus of geology at Miskatonic University and a leader of the disastrous Miskatonic University Expedition to Antarctica in 1930–31. Only his last name is mentioned in the text of Mountains, though he is fully identified in Lovecraft's "The Shadow Out of Time", where he accompanies an expedition to Australia's Great Sandy Desert.

Danforth: Graduate student at Miskatonic University. As part of the Miskatonic University Expedition, he accompanies Dyer on a survey flight over the newly discovered mountain range and goes mad after seeing something. He is described as "a great reader of bizarre material" and makes frequent allusions to Edgar Allan Poe. He is described as one of the few who ever dared to read the complete edition of the Necronomicon from the Miskatonic University Library.

Frank H. Pabodie: A member of Miskatonic's engineering department, Professor Pabodie invented a drill for the expedition that was "unique and radical in its lightness, portability, and capacity...to cope quickly with strata of varying hardness." He also added "fuel-warming and quick-starting devices" to the expedition's four aircraft.[2] Lovecraft wrote of the name "Pabodie", "I chose it as a name typical of good old New England stock, yet not sufficiently common to sound conventional or hackneyed." It's an alternative spelling of "Peabody", a name Lovecraft was familiar with through the Peabody Museum in Salem.[3]

Professor Lake: Lake is a professor of biology at Miskatonic University. It is he who first discovers the Mountains of Madness as a result of his "strange and dogged insistence on a westward - or rather, northwestward - prospecting trip" based on his discovery of strange fossils. He also discovers the ancient extraterrestrial specimens that he dubs Elder Things based on their resemblance to "certain monsters of primal myth" found in the Necronomicon. He reports that his findings in Antarctica confirm his belief "that Earth has seen whole cycles of organic life before known one that begins with Archaeozoic cells," and predicts that this "[w]ill mean to biology what Einstein has meant to mathematics and physics." When eight of the Elder Things turn out to be living creatures rather than fossils, they butcher Lake and the rest of his sub-expedition. For the rest of the story, Lake is referred to as "poor Lake".

Professor Atwood: A member of the Miskatonic University physics department, and also a meteorologist. He was part of the Lake sub-expedition.

The expedition is sponsored by the Nathaniel Derby Pickman Foundation, combining two major names in Lovecraft's fiction: Derby and Pickman.[4] Richard Upton Pickman is the main character in Lovecraft's "Pickman's Model", while Edward Pickman Derby is the protagonist of his "The Thing on the Doorstep", and also one of his literary alter-egos.[5]

Lovecraft had a lifelong interest in Antarctic exploration. "Lovecraft had been fascinated with the Antarctic continent since he was at least 12 years old, when he had written several small treatises on early Antarctic explorers," biographer S. T. Joshi wrote.[6] At about the age of 9, inspired by W. Clark Russell's 1887 book The Frozen Pirate, Lovecraft had written "several yarns" set in Antarctica.[7]

By the 1920s, Antarctica was "one of the last unexplored regions of the Earth, where large stretches of territory had never seen the tread of human feet. Contemporary maps of the continent show a number of provocative blanks, and Lovecraft could exercise his imagination in filling them in...with little fear of immediate contradiction."[8]

The first expedition of Richard E. Byrd took place in 1928-1930, the period just before the novella was written, and Lovecraft mentioned the explorer repeatedly in his letters, remarking at one point on "geologists of the Byrd expedition having found many fossils indicating a tropical past".[9] In fact, Miskatonic University's expedition was modelled after that of Byrd.[10]

In Lovecraft: A Look Behind the Cthulhu MythosLin Carter suggests that one inspiration for At the Mountains of Madness was Lovecraft's own hypersensitivity to cold, as evidenced by an incident where the writer "collapsed in the street and was carried unconscious into a drug store" because the temperature dropped from 60 degrees to 30 degrees Fahrenheit (15 degrees to -1 degree Celsius). "The loathing and horror that extreme cold evoked in him was carried over into his writing," Carter wrote, "and the pages of Madness convey the blighting, blasting, stifling sensation caused by sub-zero temperatures in a way that even Poe could not suggest."[11] S. T. Joshi has called this theory "facile."[12]

Joshi further cites as Lovecraft's most obvious literary source for At the Mountains of MadnessEdgar Allan Poe's only novel, The Narrative of Arthur Gordon Pym of Nantucket, whose concluding section is set in Antarctica. Lovecraft twice cites Poe's "disturbing and enigmatic" story in his text, and explicitly borrows the mysterious cry Tekeli-li from Poe's work. In a letter to August Derleth, Lovecraft wrote that he was trying to achieve with his ending an effect similar to what Poe accomplished in Pym.[13]

Another proposed inspiration for At the Mountains of Madness is Edgar Rice Burroughs' At the Earth's Core (1914), a novel that posits a highly intelligent reptilian race, the Mahar, living in a hollow Earth. "Consider the similarity of Burroughs' Mahar to Lovecraft's Old Ones, both of whom are presented sympathetically despite their ill-treatment of man," writes critic William Fulwiler. "[B]oth are winged, web-footed, dominant races; both are scientific scholarly races with a talent for genetics, engineering, and architecture; and both races use men as cattle." Both stories, Fulwiler points out, involve radical new drilling techniques; in both stories, humans are vivisected by nonhuman scientists. Burroughs' Mahar even employ a species of servants known as Sagoths, possibly the source of Lovecraft's Shoggoth.[14]

Other possible sources include A. Merritt's "The People of the Pit", whose description of an underground city in the Yukon bears some resemblance to that of Lovecraft's Elder Things, and Katharine Metcalf Roof's "A Million Years After", a story about dinosaurs hatching from eggs millions of years old that appeared in the November 1930 edition Weird Tales.[15] In a letter to Frank Belknap Long, Lovecraft declared Metcalf Roof's story to be a "rotten", "cheap", and "puerile" version of an idea he had come up with years earlier, and his dissatisfaction may have provoked him to write his own tale of "the awakening of entities from the dim reaches of Earth's history."[16]

Lovecraft's own "The Nameless City" (1921), which also deals with the exploration of an ancient underground city apparently abandoned by its nonhuman builders, sets a precedent for At the Mountains of Madness. In both stories, the explorers use the nonhumans' artwork to deduce the history of their species.[19] Lovecraft had also used this device in "The Dream-Quest of Unknown Kadath" (1927)

As for details of the Antarctic setting, the author's description of some of the scenery is in part inspired by the Asian paintings of Nicholas Roerich, and the illustrations of Gustave Doré, both of whom are referenced by the story's narrator multiple times.

The novella was serialized in the February, March, and April 1936 issues, and Lovecraft received $315—the most he had ever received for a story.[21] The story, however, was harshly edited, with alterations to spellings, punctuation, and paragraphing, and the end of the story had several lengthy passages omitted.[20] Lovecraft was outraged and called Tremaine "that god-damn'd dung of a hyaena [sic]".[20] Lovecraft's own hand-corrected copies of Astounding Stories formed the basis for the first Arkham House edition, but this still contained over a thousand errors, and a fully restored text was not published until 1985.[20]

In October and November 2010, BBC7 broadcast an abridged reading in five half-hour episodes performed by Richard Coyle.[25] This was repeated on BBC Radio4 Extra in March 2013, and again in August 2015.

In 2011, Cerasus Media released a hidden object game titled Mystery Stories: Mountains of Madness, with Danforth being replaced by an original female character named Lynn Morgan, who accompanies Dyer on the exploration while Danforth himself becomes the injured pilot of their aircraft and still goes insane from the experience.[30]

At the Mountains of Madness was adapted into a black metal album Tekeli-li by the band The Great Old Ones in 2014.[31]

Director Guillermo del Toro and screenwriter Matthew Robbins wrote a screenplay based on Lovecraft's story in 2006, but had trouble getting Warner Bros. to finance the project. Del Toro wrote, "The studio is very nervous about the cost and it not having a love story or a happy ending, but it's impossible to do either in the Lovecraft universe."[32] In July 2010 it was announced that the film would be made in 3D and that James Cameron would become producer,[33] and Tom Cruise was attached to star.[34] This "was a startling prospect considering Lovecraft's tale had long been considered unfilmable."[34] Del Toro confirmed that the film would begin production as early as May 2011 and start filming in June.[35] However, in March 2011, it was announced that "Universal Studios refused to greenlight the project due to del Toro's insistence that it be released with an R rating rather than a PG-13."[34] In 2012, del Toro posted that, due to the resemblance in premise with the Ridley Scott film Prometheus, the project would probably face a "long pause—if not demise".[36][37] In 2013, del Toro stated in an interview that he would try one more time to get the picture made.[38]

David A. McIntee noted similarities between the first half of the 1979 science fiction horror film Alien, particularly in early versions of the script, to At the Mountains of Madness, "not in storyline, but in dread-building mystery", and calls the finished film "the best Lovecraftian movie ever made, without being a Lovecraft adaptation", due to its similarities in tone and atmosphere to Lovecraft's works.[39] In 2009, Alien writer Dan O'Bannon said the film was "strongly influenced, tone-wise, by Lovecraft, and one of the things it proved is that you can't adapt Lovecraft effectively without an extremely strong visual style ... What you need is a cinematic equivalent of Lovecraft's prose."[40]

"A Colder War" is a loose sequel to At the Mountains of Madness written by Charles Stross in 1997, where the secrets from beyond the Mountains of Madness are used by the Cold War superpowers to dreadful effect.

Chaosium Games released a campaign book titled Beyond the Mountains of Madness for their Call of Cthulhu role-playing game in 1999. This book details the Starkweather-Moore expedition return to the ice to discover the truth about the Miskatonic Expedition. The book incorporates many of the aspects of the original Lovecraft story, including references to the Poe story, The Narrative of Arthur Gordon Pym of Nantucket, Nicholas Roerich, Danforth and Dyer.

In 2005, Elder Signs Press published Hive by Tim Curran.[43] In the story, set in the present, the Plateau of Leng has crumbled under the ice and snow due to geological changes, the Shoggoths are non-existent (with Dyer's accounts of them overthrowing the Elder Things having been chalked up to stress and madness), and the Elder Things, both living and ethereal, still exist under the Antarctic ice. The plot deals with a group of American explorers unearthing an Elder Things' tomb and citadel. A parallel plot also deals with an expedition using an experimental submersible to breach Lake Vostok, which is named as the location of the underwater city to which the Elder Things fled.

In 2009, Atlas Games published The Antarctic Express by Kenneth Hite and Christina Rodriguez, part of the Mini-Mythos line of books from Atlas Games (ISBN1589781112). It is a parody of The Polar Express, telling the story of young Danforth who boards a mysterious plane with Professor Dyer. They travel to Antarctica to the city of the Elder Things and flee when pursued by Shoggoths.